The simplest type of semiconductor radiation detector involves observing the change in conductivity in a semiconductor due to the creation of additional charge carriers by an incident electromagnetic field or ionization created by a charged particle. These devices are commonly known as photoconductors. Semiconductor junction photodiodes can exhibit dramatically improved performance over simple photoconductor detectors. The most common form of photodiode used is the silicon PIN diode in which a thick layer of an intrinsic semiconductor is grown between the n and p layers of the junction. The same structure can be made in germanium or any compound semiconductor. PIN diodes are normally fabricated using n-type silicon substrates with a bulk resistivity >1,000 Ohm-cm.
These PIN diodes normally work under full depletion. The depletion region is devoid of free thermal carriers. Hence, it is necessary to ensure the operation of PIN sensors at high voltages, where the PIN sensors are limited by the breakdown phenomena. Breakdown of these sensors is generally caused by the electric field enhancement at the edges and corners. Since these sensors are made on silicon wafers, individual devices need to be separated. This process is called dicing. Dicing is normally done by using a diamond blade, which generates mechanical damage on the cut surface. This damage leads to a damaged silicon region that shows electrical conductivity and promotes electric breakdowns.
Modern radiation detectors are based on high-voltage semiconductor devices. They are often segmented for position sensitive detection. The semiconductor substrates are commonly silicon, germanium, III-VI, or II-VI compound semiconductor. The basic operation is similar to a solar cell. The radiation hits the detector and generates charge carriers, which are then collected on either top or bottom surface of the device. In order to achieve good charge collection from the full depth of the detector, voltages up to several thousands volts are applied. Charge collection only happens in the active area of the devices; the border regions are non-active. A guard ring or multiple guard rings separate the active from the border regions. Furthermore, the guard ring protects to active region of the device from the damage from the dicing. Since the size of a single device is limited (max. by the wafer size), larger detector arrays are formed using tiled semiconductor devices. Sensor tiling or other techniques lead to detectors systems with large instrumented surfaces. Since border regions of each individual device are still inactive, one wants to minimize these dead regions. Outside of the guard rings there is often an implant region, which, through the conductive edge of the cut, brings the backside potential to the top periphery of the detector. This limits the current which otherwise might flow through the cut edge containing large defect density.
In some cases, the area in the immediate vicinity to the edge on either top or bottom surface has an additional implant to keep the field gradient away from the conductive edge. Neither the guard ring structure nor the implant area contributes to collection of electrical charge.
The standard approach to form an active edge is to micro-fabricate a trench around the device. The trench micro-fabrication is done by silicon reactive ion etching (RIE). RIE has become more and more common for the fabrication of high-aspect ratio structures in silicon. RIE etching is sometime called dry silicon etching in comparison to wet chemical or electrochemical etching. RIE is done in plasma, ions are accelerated towards the material to be etched, and the etching reaction is enhanced in the direction of the ion. RIE etching is a single wafer process; making this process very time-consuming and costly. The etching of high-aspect ratio structures in silicon is done by using high density, inductively coupled plasma (ICP) and fluorine-ion based chemistry. ICP-plasmas give very high plasma and radical densities and low substrate bias, which allow the formation of high-aspect ratio structures with no crystal damage. This technique is called DRIE (deep reactive ion etching). After the trench micro-fabrication the trench is oxidized or an active junction is formed. An active junction is based on a p-n junction within the silicon.
What is needed is a method for reducing the inactive area, minimizing the number of needed guard rings or removing them and eliminating the need of using an implant to detectors.